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European Frequency and Time Forum

Hyperfine coherence and population lifetime in Cs vapor cells

Jean-Marie Danet, Olga Kozlova, Stéphane Guérandel, Emeric de Clercq

Published Apr 1, 2012• 0 citations
Publisher

Abstract

Within the frame of a Raman-Ramsey CPT clock optimisation, we present our investigations on the coherence and population lifetime measurements in Cs vapour cells. To guarantee the reliability of the coherence lifetime measurements, three different methods are compared. Very good agreement is found between two of them and results are discussed for the third. In order to increase the coherence lifetime, two techniques are tested: Firstly a buffer gas can be added in the Cs vapour cell and secondly cell walls can be coated to reduce the destructive collisions against the walls. Buffer gas is tested first and then combined to a coating. First results are presented.

DOI
10.1109/eftf.2012.6502343

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Within the frame of a Raman-Ramsey CPT clock optimisation, we present our investigations on the coherence and population lifetime measurements in Cs vapour cells. To guarantee the reliability of the coherence lifetime measurements, three different methods are compared. Very good agreement is found between two of them and results are discussed for the third. In order to increase the coherence lifetime, two techniques are tested: Firstly a buffer gas can be added in the Cs vapour cell and secondly cell walls can be coated to reduce the destructive collisions against the walls. Buffer gas is tested first and then combined to a coating. First results are presented.
Pulsed coherent population trapping (CPT) spectroscopy is applied in buffer-gas filled Cs–Ne vapor microfabricated cells. The properties of the Ramsey-CPT clock transition central fringe (linewidth, signal amplitude, and contrast) are studied versus several experimental parameters, including the Ramsey sequence and the input laser intensity, and are compared to those obtained in the conventional continuous (CW) interrogation mode. In the pulsed case and for short Ramsey-free evolution times TR, the central fringe linewidth is found to exhibit a small but visible power broadening and is measured to be narrower than the value of 1/(2TR) that is reached for long TR. The microwave hyperfine coherence lifetime T2 in the Cs–Ne microcells is measured to be in the 50–500 μs range. Its dependence to the buffer gas pressure or cell temperature is briefly studied.
We report the realization and characterization using coherent population trapping (CPT) spectroscopy of an octadecyltrichlorosilane (OTS)-coated centimeter-scale Cs vapor cell. The dual-structure of the resonance lineshape, with presence of a narrow structure line at the top of a Doppler-broadened structure, is clearly observed. The linewidth of the narrow resonance is compared to the linewidth of an evacuated Cs cell and of a buffer gas Cs cell of similar size. The Cs-OTS adsorption energy is measured to be (0.42 ± 0.03) eV, leading to a clock frequency shift rate of 2.7 × 10−9/K in fractional unit. A hyperfine population lifetime, T1, and a microwave coherence lifetime, T2, of 1.6 and 0.5 ms are reported, corresponding to about 37 and 12 useful bounces, respectively. Atomic-motion induced Ramsey narrowing of dark resonances is observed in Cs-OTS cells by reducing the optical beam diameter. Ramsey CPT fringes are detected using a pulsed CPT interrogation scheme. Potential applications of the Cs-OTS cell to the development of a vapor cell atomic clock are discussed.
We report the characterization using coherent population trapping (CPT) spectroscopy of an octadecyltrichlorosilane (OTS)-coated centimeter-scale Cs vapor cell. The linewidth of the narrow CPT resonance is compared to the linewidth of an evacuated Cs cell of similar size. The Cs-OTS adsorption energy is measured to be (0.42 ± 0.03) eV. A hyperfine population lifetime, T1of 1.6 ms is reported, corresponding to about 37 useful bounces. Ramsey CPT fringes are detected using a pulsed CPT interrogation scheme.
We report the realization and characterization using coherent population trapping (CPT) spectroscopy of an octadecyltrichlorosilane (OTS)-coated centimeter-scale Cs vapor cell. The dual-structure of the resonance lineshape, with presence of a narrow structure line at the top of a Doppler-broadened structure, is clearly observed. The linewidth of the narrow resonance is compared to the linewidth of an evacuated Cs cell and of a buffer gas Cs cell of similar size. The Cs-OTS adsorption energy is measured to be (0.42 ±\pm± 0.03) eV, leading to a clock frequency shift rate of 2.7×10−9/2.7\times10^{-9}/2.7×10−9/K in fractional unit. A hyperfine population lifetime, T1T_1T1​, and a microwave coherence lifetime, T2T_2T2​, of 1.6 and 0.5 ms are reported, corresponding to about 37 and 12 useful bounces, respectively. Atomic-motion induced Ramsey narrowing of dark resonances is observed in Cs-OTS cells by reducing the optical beam diameter. Ramsey CPT fringes are detected using a pulsed CPT interrogation scheme. Potential applications of the Cs-OTS cell to the development of a vapor cell atomic clock are discussed.
A gas cell atomic frequency standard based on coherent population trapping (CPT) with Ramsey interrogation pulses is developed. The clock resonance is detected in a Cs cell with buffer gas using two phase-locked diode lasers tuned to the D1 line. A preliminary short-term stability of3.5×10−12t−1/23.5 \times 10^{-12}t^{-1/2}3.5×10−12t−1/2is reported. The CPT specificity allows a new interrogation–detection sequence, i.e., the continuous pulse train, which, with phase modulation, could lead to better short-term stability compared to usual continuous CPT clocks. The parameters optimizing the stability include a high Rabi frequency, a very short measurement time, and an interrogation time equal to the lifetime of the hyperfine coherence.
Vapor cell atomic clocks exhibit reduced frequency stability for averaging time between about one hundred and a few thousand seconds. Here we report a study on the impact of the main parameters on the mid-to-long term instability of a buffer-gas vapor cell Cs clock, based on coherent population trapping (CPT). The CPT signal is observed on the Cs D1 line transmission, using a double Λ scheme and a Ramsey interrogation technique. The effects on the clock frequency of the magnetic field, the cell temperature, and the laser intensities are reported. We show in particular that the laser intensity shift is temperature dependent. Along with the laser intensity ratio and laser polarization properties, this is one of the most important parameters.